Morphologically and size uniform monodisperse particles and their shape-directed self-assembly

ABSTRACT

Monodisperse particles having: a single pure crystalline phase of a rare earth-containing lattice, a uniform three-dimensional size, and a uniform polyhedral morphology are disclosed. Due to their uniform size and shape, the monodisperse particles self assemble into superlattices. The particles may be luminescent particles such as down-converting phosphor particles and up-converting phosphors. The monodisperse particles of the invention have a rare earth-containing lattice which in one embodiment may be an yttrium-containing lattice or in another may be a lanthanide-containing lattice. The monodisperse particles may have different optical properties based on their composition, their size, and/or their morphology (or shape). Also disclosed is a combination of at least two types of monodisperse particles, where each type is a plurality of monodisperse particles having a single pure crystalline phase of a rare earth-containing lattice, a uniform three-dimensional size, and a uniform polyhedral morphology; and where the types of monodisperse particles differ from one another by composition, by size, or by morphology. In a preferred embodiment, the types of monodisperse particles have the same composition but different morphologies. Methods of making and methods of using the monodisperse particles are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. application Ser. No.14/878,424, filed Oct. 8, 2015; which is a Continuation of applicationSer. No. 13/876,266, filed Jun. 24, 2013, now U.S. Pat. No. 9,181,477;which claims priority to International PCT International Application No.PCT/US2011/054593, filed Oct. 3, 2011; which claims priority toApplication No. 61/388,941, filed Oct. 1, 2010; to U.S. Application61/487,784, filed May 19, 2011; and to U.S. Application 61/487,785,filed May 19, 2011; which are all incorporated herein by reference.

BACKGROUND OF THE INVENTION

Recent advances in synthesis and controlled assembly of monodispersecolloidal nanocrystals into superlattice structures have enabled theirapplications in optics (1), electronics (2), magnetic storage (3), solarenergy, etc. Single- and multi-component superlattices composed ofspherical nanocrystals have been extensively studied in a variety ofaspects such as structural diversity (4, 5), and electronic (6) andmagnetic (7) interactions between the constituents. On the other hand,significant efforts have been put into developing new syntheticapproaches for non-spherical nanocrystals that often exhibit physicalproperties unobtainable by simply tuning the size of the particles(8-11). However, the organization and application of these anisotropicbuilding blocks have been limited mainly due to the lack of sufficientcontrol over size uniformity, shape selectivity, surface functionalityand the scarcity of convenient and reliable assembly methodology.

One particularly interesting class of materials which have widepotential as monodisperse colloidal nanocrystals are inorganicluminescent or electromagnetically active materials, crystallinematerials that absorb energy acting upon them and subsequently emit theabsorbed energy. Light emission is known as luminescence. A luminescentmaterial which continues to emit light for greater than 10⁻⁸ secondsafter the removal of the absorbed light is said to be phosphorescent.Phosphorescent substances are also known as phosphors. The half-life ofthe afterglow, or phosphorescence, of a phosphor will vary with theparticular substance and typically ranges from about 10⁻⁶ seconds todays. Phosphors may generally be categorized as stokes (down-converting)phosphors or anti-stokes (up-converting) phosphors. Phosphors whichabsorb energy in the form of a photon and emit a lower frequency (lowerenergy, longer wavelength) band photon are down-converting phosphors. Incontrast, phosphors which absorb energy in the form of two or morephotons in a low frequency and emit in a higher frequency (higherenergy, shorter wavelength) band are up-converting phosphors.Up-converting phosphors, for example, are irradiated by near infra-redlight, a lower energy, longer wavelength light, and emit visible lightwhich is of higher energy and a shorter wavelength. Phosphors may alsobe categorized according to the nature of the energy which excites thephosphor. For example, phosphors which are excited by low energy photonsare called photoluminescent and phosphors which are excited by cathoderays are called cathodluminescent.

Lanthanide-doped nanophosphors have become an emerging class of opticalmaterials during the past few years (12). These nanophosphors oftenpossess “peculiar” optical properties (e.g., quantum cutting (13) andphoton upconversion (14)), allowing the management of photons that couldbenefit a variety of areas including biomedical imaging (15, 16) andtherapy (17), photovoltaics (13, 18), solid state lightning (19), anddisplay technologies (20). Colloidal upconversion nanophosphors (UCNPs)are capable of converting long-wavelength near-infrared excitation intoshort-wavelength visible emission through the long-lived, metastableexcited states of the lanthanide dopants (21). In contrast to theStokes-shifted emissions from semiconductor nanocrystals or organicfluorophores and the multiphoton process employing fluorescent dyes,UCNPs offer several advantages including narrow, tunable emission bands(22), non-blinking emission and remarkable photostability (15, 23), goodbrightness under low power continuous wave laser excitation, lowautofluorescence background and deep penetration depths in biologicalsystems (15, 16), etc. It has been widely accepted that hexagonal phaseNaYF₄ (β-NaYF₄) is one of the best host materials for upconversion dueto its low phonon energies (24), being more efficient than the cubic,α-NaYF₄ phase (25). Several chemical approaches includingcoprecipitation (26) and hydrothermal synthesis (27) have been employedto synthesize β-NaYF₄-based UCNPs. However, these methods are usuallylimited by drawbacks such as the necessity of post-synthesis treatmentto improve crystallinity of the products, long reaction time (rangingfrom a few hours up to several days) and the use of specialized reactors(e.g., autoclaves). The synthesis of lanthanide fluoride nanocrystalsvia the thermal decomposition of metal trifluoroacetate precursors hasbeen described (28, 29). Preparations of β-NaYF₄-based UCNPs throughdecomposition of mixed trifluoroacetates (30) or through a two-stepripening process using the premade α-NaYF₄ nanocrystals as precursors(31) were subsequently reported. Other reported methods to preparemonodisperse inorganic phosphor particles include sol-gel methods (U.S.Pat. No. 5,637,258); fluidized bed methods (U.S. Pat. No. 6,039,894);and solution-precipitation of precursors followed by heating (U.S. Pat.No. 6,132,642). Despite these recent progresses, the feasibility of thesynthetic approach and the quality of the as-synthesized UCNPs or otherinorganic particles using existing recipes are still far fromsatisfactory. There remains a need for a straightforward synthesis ofnot only UCNPs but for luminescent or electromagnetically activeinorganic particles in general.

UCNPs, as one example, and other inorganic phosphor particles as well,are employed in a wide variety of applications, for example, inlabeling, 3-D volumetric displays and in diagnostic assays. See, e.g.,U.S. Pat. Nos. 4,870,485; 5,943,160; 5,043,265; 5,698,397; and7,858,396; and U.S. Published Application 2007/0247595. Upconverting anddownconverting phosphors are also used in smaller niche areas such ascosmetic/cosmeceutical, printed art, vehicle automation and guidance,process control, among others.

Inorganic phosphors are used, for example, to label security documentsor currency. Phosphor particles may be incorporated into a document,currency or other security articles and detected to determine itsauthenticity. Phosphors used in this way are often called “taggants.”Combinations of different phosphor particles with different excitationand/or emission wavelengths, are used to provide a unique securitysignature to the security document or currency, etc. See U.S. Pat. Nos.7,030,371; 7,790,056; 7,927,511; and 7,999,237. Similarly, phosphorparticles may be incorporated into bulk materials, such as rawingredients, and provide a label or signature that identifies the sourceand/or integrity of the bulk material. See, e.g., U.S. Pat. Nos.7,323,696 and 6,536,672.

Diagnostic assays employ phosphor particles as detection labels to showthe presence or absence of a particular analyte of interest. Thephosphor particles may be surface treated to enable them to bind to thebiological components of the assay and/or the analyte of interest. Usingphosphor particles with different excitation and/or emission wavelengthsallow for multiplexing—the detection of more than one analyte in asingle assay using different labels to identify particular analytes.See, e.g., U.S. Pat. Nos. 5,043,265; 5,698,397; and 7,858,396. Phosphorparticles find numerous uses in in vitro diagnostics, includingpoint-of-care diagnostics, flow cytometry, high throughput screening fordrug discovery, genetic analyses, or early detection of disease.Generally, the types of analyses in this area encompass; state ofhealth, congenital diseases, progress or course of treatment, and butnot limited to determination of compatibility in blood or organdonations and transplants and labeling of cellular markers for use inpathological analysis of specimens. There are applications across manyfields of medicine for medical imaging in the diagnosis and tracking ofdisease and in therapeutics such as Photodynamic Therapy, a light-basedcancer therapy used for various malignancies. For example, phosphorparticles are also used in biomedical imaging and therapy where thephosphors attach to tissue or cells for use in both in vitro and in vivotesting as well as in treatment of disease such as cancer See, e.g.,Xiong L, Yang T, Yang Y, Xu C, Yi F (2010) Biomaterials, 7078-7085, andUngun B, Prud'homme R, Budijono S, et al. (2008) Optics Express 17(1),80-86.

The differentiation among phosphors or combination of differentphosphors as labels is dependent on the use of a combination ofdifferent phosphor based on phosphor composition. As described in U.S.Pat. No. 5,698,397, phosphors may use different combination of absorbersand emitters to create unique phosphors. For example, a plurality ofphosphors may absorb the same wavelength of light but, by havingdifferent emitters, emit at different wavelengths. Alternatively, aplurality of phosphors may emit the same wavelength but absorb differentwavelengths. Of course, the phosphors in a plurality of phosphors mayeach have its own unique absorption wavelength and unique emissionwavelength. Thus, based on the particular phosphor composition,different absorption and/or emission profiles are possible. Thiscompositional differentiation is needed when the phosphor particles arespherical or do not have a uniform shape distribution. This, in turns,places demands and constraints on the equipment used when combinationsof phosphors are used to create a unique signature or to differentiateamong analytes.

SUMMARY OF THE INVENTION

In one embodiment, the invention relates to a plurality of monodisperseparticles having: a single pure crystalline phase of a rareearth-containing lattice, a uniform three-dimensional size, and auniform polyhedral morphology. Due to their uniform size and shape, themonodisperse particles self assemble into superlattices, which representa further embodiment of the invention. The particles may be luminescentparticles such as down-converting phosphor particles and up-convertingphosphors and have the utilities known for such phosphors. In anotherembodiment of the invention, the monodisperse particles are crystallinenanoparticles. The monodisperse particles of the invention have a rareearth-containing lattice which in one embodiment may be anyttrium-containing lattice or in another may be a lanthanide-containinglattice. The various monodisperse particle compositions and morphologiesidentified below each represent a separate embodiment of the invention.

The monodisperse particles of the invention may have different opticalproperties based on their composition, their size, and/or theirmorphology (or shape). In another embodiment, the invention relates to acombination of at least two types of monodisperse particles, where eachtype is a plurality of monodisperse particles having a single purecrystalline phase of a rare earth-containing lattice, a uniformthree-dimensional size, and a uniform polyhedral morphology; and wherethe types of monodisperse particles differ from one another bycomposition, by size, or by morphology. In a preferred embodiment, thetypes of monodisperse particles have the same composition but differentmorphologies.

The invention also relates to a method for the preparation ofmonodisperse particles of the invention. The method employs the stepsof: in a reaction vessel, dissolving at least one precursor metal saltin a solvent to form a solution; placing the reaction vessel in a heatedsalt bath having a temperature of at least about 340° C.; applying heatto the salt bath to rapidly decompose the precursor metal salts in thesolution to form the monodisperse particles; keeping the reaction vesselin the salt bath for a time sufficient to increase the size of themonodisperse particles; removing the reaction vessel from the salt bath;and quenching the reaction with ambient temperature solvent.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the rise/decay times of Yttrium Oxide upconverting phosphorparticles of the invention having varying Ytterbium/Erbium ratios.

FIG. 2 shows the lifetimes of various morphologies and SurfaceArea/Volume (SA/Vol) ratios of NaYF₄ hexagonal prisms of the inventionat an excitation wavelength was 980 nm, Emission Lifetime read at 543nm.

FIGS. 3A-3L show TEM images of the β-NaYF₄-based upconvertingnanophosphor particles of the invention.

FIGS. 4A-4G show the structural and optical characterization of theβ-NaYF₄-based upconverting nanophosphor particles of the invention.

FIGS. 5A-5D show NaYF₄ (AR=1.4) nanorod superlattices of the invention.

FIGS. 6A-6D show NaYF₄ (AR=2.0) nanorod superlattices of the invention.

FIGS. 7A-7B show hexagonal nanoprism and nanoplate superlattices of theinvention.

FIGS. 8A-8C show the unique morphologies in the NaYF₄:Yb, Er particlesof the invention.

FIGS. 9A-9C show the unique morphologies in the LiYF₄:Yb, Er particlesof the invention.

FIG. 10A shows TEM images of LiYF₄:Yb,Er tetragonal bi-pyramid particlesat ˜150 nm, and FIG. 10B shows TEM images of the LiYF₄:Yb,Er diamondparticles at ˜75 nm.

FIG. 11 shows a TEM image of EuF₃ particles of the invention.

FIG. 12 shows the visible red emission (˜610 nm) and associatedabsorption and emission spectra of Eu F₃ nanoparticles under UVexcitation.

FIG. 13 shows a TEM image of NaGdF₄:Tb spherical hexagon particles ofthe invention.

FIG. 14 is the emission spectra from NaGdF₄:Tb doped nanoparticles under6 megavolt X-Ray radiation. The overlaid spectrum is the absorption of aprotoporphyrin photosensitizer utilized for Photodynamic Therapy.

DETAILED DESCRIPTION

The invention relates to monodisperse particles having a single purecrystalline phase of a rare earth (RE)-containing lattice, a uniformthree-dimensional size, and a uniform polyhedral morphology. RE elementsinclude yttrium and the elements of the lanthanide (Ln) series, i.e.,lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Ne),promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium (Lu). Having uniform three-dimensional sizeand uniform polyhedral morphology, the monodisperse particles of theinvention have the same dimensions and the same shape, within atolerance of about 10 nm or less. Depending on their composition,monodisperse particles of the invention may have crystal symmetries of,but not limited to, tetragonal bipyramids, hexagonal prisms, rods,hexagonal plates, ellipsoids, trigonal prisms, and triangular plateswhich determine the uniform polyhedral morphologies of the particularparticles. Being of uniform three-dimensional size and uniformpolyhedral morphology allows the particles to uniformly orientthemselves in a three-dimensional pattern having the lowest free energystate for the assemblage. In other words, the monodisperse particles ofthe invention advantageously self-assemble or “tile” to form superlattices. These superlattices are themselves another embodiment of theinvention, discussed below.

The monodisperse particles of the invention may vary in size. In oneembodiment, particles of the invention may be described as nanocrystalsor colloidal particles with their largest dimension being approximately1 nm to 1,000 nm in size. Large particles, with at least one dimensionof approximately 1 μm to 250 μm, represent another embodiment of theinvention. The size of the particle depends on the stoichiometric ratioof elements making the particle or the stoichiometric ratio precursorused to prepare the particle as well as the length of reaction time,discussed below.

Monodisperse particles of the invention have a single pure crystallinephase of a RE-containing lattice (e.g., an yttrium-containing lattice ora lanthanide-containing lattice). The lattice contains yttrium (Y) or alanthanide (Ln) in its +3 oxidation state. The charge is balanced in thelattice by the presence of an anion such as a halide (fluoride, F⁻,being preferred), an oxide, an oxysulfide, an oxyhalide (e.g., OCl), asulfide, etc. Alkali metals, i.e., lithium (Li), sodium (Na), potassium(K), rubidium (Rb), and cesium (Cs) and/or alkali earth metals beryllium(Be), magnesium (Mg) calcium (Ca), strontium (Sr), and barium (Ba) mayalso be a component of the host lattice. The alkali metals or alkalineearth metals are often called “lattice modifiers.” For the synthesis ofmonodisperse particles of the invention, the alkali metal or alkalineearth metal present in the lattice may determine the crystal symmetryproviding morphological control over the particles as well asindependent tunability of a particle's other properties, such as theoptical properties of a luminescent particle. For example, the crystalsymmetry of LiYF₄, NaYF₄, and KYF₄ are tetragonal, hexagonal, andtrigonal, respectively. The chemical composition of the particles of theinvention provides unique polyhedral morphologies. Representativeyttrium-containing lattices include, but are not limited to LiYF₄,BaYF₅, BaY₂F₃ NaYF₄, KYF₄, Y₂O₂S, Y₂O₃, and the like. Thelanthanide-containing lattice may be one having any element of thelanthanide series. Representative lanthanide-containing latticesinclude, but are not limited to, LaF₃, CeF₃, PrF₃, NeF₃, Pm F₃, SMF₃,EuF₃, GdF₃, Tb F₃, DyF₃, HoF₃, ErF₃, Tm F₃, YbF₃ Lu F₃, NaGdF₃, Gd₂OS₂,LiHoF₄, LiErF₄, CeO, SrS, CaS, GdOCl, and the like. Compositions havingsuch a lattice are known in the art to be inorganic luminescentmaterials and/or electromagnetically active materials. As known in theart, these compositions typically contain dopants and lattice modifiers,discussed below, that impart unique properties to the composition. Asdescribed below, the methods of the invention may be used to preparemonodisperse particles according to the invention having thosecompositions.

As discussed above, the luminescent materials are typically phosphors.Both up-converting phosphors and down-converting phosphors are known forRE-containing lattices. Compositions having a RE-containing lattice arealso known to have electromagnetic properties and are useful assemiconductors. Exemplary compositions and their properties arediscussed below. According to the invention, particles having thosecompositions may be prepared such that the particles are particles ofthe invention-monodisperse particles having a single pure crystallinephase of an yttrium-containing or a lanthanide-containing lattice, auniform three-dimensional size, and a uniform polyhedral morphology.

Luminescent Inorganic Materials (Phosphors)

An inorganic phosphor comprises a crystalline structure known in the artas a host lattice, which the host lattice can be combined with a lightemitting dopant. Such host lattice structures and host lattice-dopantcombinations are commonly known in the art. Monodisperse particles ofthe invention have a single pure crystalline phase of RE-containinglattice, e.g., an yttrium-containing or a lanthanide-containing latticeas the host lattice. A “dopant” is a substance that absorbs primarylight energy originating from the light source and emits secondary lightof a secondary wavelength in response to said primary light energy. Whenused in combination with a host lattice, the dopant typically is anelemental substitute in the host lattice crystal, serving as asubstitute for another element. The element being replaced depends onthe composition of the host lattice. For the yttrium-containing orlanthanide-containing lattices used in the particles of the invention,the dopant is often an RE metal, quite often a lanthanide or combinationof lanthanides, such as Y, Gd, and La (although the dopant is differentfrom the RE in host lattice). The dopant element is generally of thesame charge and also generally at a small level compared to the elementthat it is replacing. For example, in a host lattice-dopant combinationof NaYF₄:Yb,Er, ytterbium and erbium are the dopants and NaYF₄ is thehost lattice material. The ytterbium ions and erbium ions aresubstituted in for the yttrium ions in the host lattice material. In ahost lattice-dopant combination, the phosphor generally substitutesanother element for one in the host lattice in a small percentage thatgives optical emission properties. A phosphor serving this purpose cancomprise a single dopant or can comprise multiple dopants, and one ofthe dopants might act as a sensitizer. When present, a sensitizer ion isthe primary absorber for the phosphor, but is not the main emitter. Theenergy that the sensitizer absorbs is transferred to the main activeemitter ion (main dopant) through non-radiate transfer.

Down-Converting Phosphors

Down-converting inorganic phosphors are well known in the art. Thesematerials can absorb X-Ray to UV energies and convert to lower energyvisible or infrared wavelengths. For example, down-converting phosphormaterials include RE element doped oxides, RE element doped oxysulfides,RE element doped fluorides. Examples of down-converting phosphorsinclude, but are not limited to Y₂O₃: Gd, Y₂O₃:Dy, Y₂O₃:Tb, Y₂O₃:Ho,Y₂O₃:Er, Y₂O₃:Tm, Gd₂O₃:Eu, Y₂O₂S:Pr, Y₂O₂S:Sm, Y₂O₂S:Eu, Y₂O₂S:Tb,Y₂O₂S:Ho, Y₂O₂S:Er, Y₂O₂S:Dy, Y₂O₂S:Tm, Y₂O₂S:Eu (red), Y₂O₃:Eu (red),and YVO₄:Eu (red). Other examples of down-converting phosphors aresodium gadolinium fluorides doped with other lanthanides, e.g.,NaGdF₄:Tb, wherein the Tb can be replaced with Eu, Dy, Pr, Ce, etc.Lanthanide fluorides are also known as down-converting fluorides, e.g.,TbF₃, EuF₃, PrF₃, and DyF₃

Up-Converting Phosphors

A large variety of up-converting inorganic phosphor compositions arealso known in the art. As is known in the art, up-converting phosphorsderived from RE-containing host lattices, such as described above, dopedwith at least one activator couple comprising a sensitizer (also knownas an absorber) and an emitter. Suitable up-converting phosphor hostlattices include: sodium yttrium fluoride (NaYF₄), lanthanum fluoride(LaF₃), lanthanum oxysulfide, RE oxysulfide(RE₂O₂S), RE oxyfluoride(RE₄O₃F₆), RE oxychloride (REOCl), yttrium fluoride (YF₃), yttriumgallate, gadolinium fluoride (GdF₃), barium yttrium fluoride (BaYF₅,BaY₂ F₈), and gadolinium oxysulfide, wherein the RE can be Y, Gd, La, orother lanthanide elements. Suitable activator couples are selected from:ytterbium/erbium, ytterbium/thulium, and ytterbium/holmium. Otheractivator couples suitable for up-conversion may also be used. Bycombination of RE-containing host lattices with just these threeactivator couples, at least three phosphors with at least threedifferent emission spectra (red, green, and blue visible light) areprovided. Generally, the absorber is ytterbium and the emitting centercan be selected from: erbium, holmium, terbium, and thulium; however,other up-converting phosphor particles of the invention may containother absorbers and/or emitters. The molar ratio of absorber:emittingcenter is typically at least about 1:1, more usually at least about 3:1to 5:1, preferably at least about 8:1 to 10:1, more preferably at leastabout 11:1 to 20:1, and typically less than about 250:1, usually lessthan about 100:1, and more usually less than about 50:1 to 25:1,although various ratios may be selected by the practitioner on the basisof desired characteristics (e.g., chemical properties, manufacturingefficiency, excitation and emission wavelengths, quantum efficiency, orother considerations). For example, increasing the Yb concentrationslightly alters the absorption properties, which is useful forbiomedical applications. The phosphor particle of the invention can beexcited at 915 nm instead of 980 nm where the water absorption is muchhigher and more tissue heating occurs. The ratio(s) chosen willgenerally also depend upon the particular absorber-emitter couple(s)selected, and can be calculated from reference values in accordance withthe desired characteristics. It is also possible to control overparticle morphologies by drastically changing the ratio of theactivators without the emission properties changing drastically for mostof the ratios but quenching may occur at some point.

The optimum ratio of absorber (e.g., ytterbium) to the emitting center(e.g., erbium, thulium, or holmium) varies, depending upon the specificabsorber/emitter couple. For example, the absorber:emitter ratio forYb:Er couples is typically in the range of about 20:1 to about 100:1,whereas the absorber:emitter ratio for Yb:Tm and Yb:Ho couples istypically in the range of about 500:1 to about 2000:1. These differentratios are attributable to the different matching energy levels of theEr, Tm, or Ho with respect to the Yb level in the crystal. For mostapplications, up-converting phosphors may conveniently comprise about10-30% Yb and either: about 1-2% Er, about 0.1-0.05% Ho, or about0.1-0.05% Tm, although other formulations may be employed.

Some embodiments of the invention employ inorganic phosphors that areoptimally excited by infrared radiation of about 950 to 1000 nm,preferably about 960 to 980 nm. For example, but not by limitation, amicrocrystalline inorganic phosphor of the formula YF₃:Yb_(0.10)Er_(0.01) exhibits a luminescence intensity maximum at an excitationwavelength of about 980 nm. Up-converting phosphors of the inventiontypically have emission maxima that are in the visible range. Forexample, specific activator couples have characteristic emissionspectra: ytterbium-erbium couples have emission maxima in the red orgreen portions of the visible spectrum, depending upon the phosphorhost; ytterbium-holmium couples generally emit maximally in the greenportion, ytterbium-thulium typically have an emission maximum in theblue range, and ytterbium-terbium usually emit maximally in the greenrange. For example, Y_(0.80) Yb_(0.19) Er_(0.01)F₂ emits maximally inthe green portion of the spectrum.

Particle Properties Based on Composition, Morphology, and Size

Properties of the monodisperse particles can be tuned in a variety ofways. As known in the art and discussed above, the properties of themonodisperse particles, the characteristic absorption and emissionspectra, may be tuned by adjusting their composition, e.g., by selectinga host lattice, and/or by doping. Additionally and advantageously, giventheir uniform polyhedral morphology, the monodisperse particles of theinvention exhibit anisotropic properties. Particles of the samecomposition but different shape exhibit different properties due totheir shape and/or size. In one exemplary embodiment of the invention,the monodisperse particles, particularly UCNP's, of the invention arevaried in composition and/or shape to give different decay lifetimes.Having different spectral decay lifetimes allows unique phosphorparticles to be differentiated from one another. The ability to havemonodisperse particles of the same composition but differentmorphologies according to the invention permits use of one composition(especially in regulated industries such as pharmaceuticals or medicaldevices) but to distinguish its morphologies through their uniqueoptical properties. However, to take advantage of this, to tune theparticle and its optical properties in this way, has not been possiblebut is now achieved with the monodisperse particles of the invention.

Thus, in addition to the characteristic absorption and emission spectrathat can be obtained the rise and decay times of a monodisperse particleof the invention can also be tuned by particle size and morphology. Therise time is measured from the moment the first excitation photon isabsorbed to when the first emission photon is observed. The decay timeis measured by the slope of the emission decay, or the time it takes forthe phosphor to stop emitting once the excitation source is turned off.This is also described as the time it takes for depletion of electronsfrom the excited energy levels. FIG. 1 shows the rise and decay timetunability of a Y₂O₃:Yb,Er doped phosphor particles of the inventionexcited at 1540 nm. By changing the dopant ratio, the rise and decaytimes can be reliably altered.

Briefly, typically an excited state population decays exponentiallyafter turning off the excitation pulse by first-order kinetics,following the decay law, I(t)=I₀exp (−t/τ), whereby for a singleexponential decay I(t)=time dependent intensity, I₀=the intensity attime 0 (or amplitude), and τ=the average time a phosphor (or fluorophor)remains in the excited state (or <t>) and is equal to the lifetime. (Thelifetime τ is the inverse of the total decay rate, τ=(T+k_(nr))⁻¹, whereat time t following excitation, T is the emissive rate and k_(nr) is thenon-radiative decay rate). In general, the inverse of the lifetime isthe sum of the rates which depopulate the excited state. Theluminescence lifetime can be simply determined from the slope of a plotof InI(t) versus t (equal to 1/τ). It can also be the time needed forthe intensity to decrease to 1/e of its original value (time 0). Thus,for any given known emission wavelength, a number of parameters fittingthe exponential decay law can be monitored to identify a particularphosphor or group of phosphors, thus permitting their use, for example,in developing unique anti-counterfeiting codes, signatures, orlabels/taggants.

In most instances, lifetimes are controlled by variations in the crystalcomposition or overall particle size. However, by controlling theparticle morphology and uniformity as with the monodisperse particles ofthe invention one can create particles of visually distinct morphologiespossessing lifetimes that are unique to that morphology whilemaintaining identical chemical compositions among the variousmorphologies. This feature allows for a highly complex optical signatureor taggant which, as discussed above, may be used in serialization andmultiplexing assays or analysis in various fields such as, for example,assays, biomedical, optical computing, as well as use in security andauthentication.

To illustrate, the dependences of upconversion luminescence (UCL) on theparticle size, shape, and inorganic-ligand interface of the hexagonal(β)-phase NaYF₄:Yb,Er upconverting nanophosphors of the invention wereinvestigated (see Shan J, Uddi M, Wei R, Yao N, Ju Y. (2010) J. Phys.Chem 114, 2452-2461). The relative luminescent intensity,power-dependent luminescence, green to red emission intensity ratio(fg/r), and dynamic luminescence lifetimes of the prism-, plate-, androd-shaped hexagonal (β)-phase NaYF₄:Yb,Er particles of the invention asa function of surface to volume (SA/Vol) ratio was measured. Theupconverting properties of the particles can be attributed to not onlythe surface effects by comparing the SA/Vol ratios but also the particlemorphologies or shapes. At the comparable SA/Vol and ion (Yb/Er) dopingratios (20%/2%), the prism-shaped nanocrystal particles showed increasedintensity and smaller saturation power than those of the rod-shapednanocrystals. Therefore, the differently shaped nanocrystals withidentical SA/Vol ratios could have different lattice energy andmultiphonon relaxation processes, such as shown in FIG. 2. The lifetimesof various morphologies and Surface Area/Volume (SA/Vol) ratios of NaYF₄hexagonal prism particles for the invention are shown in FIG. 2. Theexcitation wavelength was 980 nm and the emission lifetime was read at543 nm.

Methods for the Preparation of the Monodisperse Particles of theInvention.

The invention also relates to a method for the preparation ofmonodisperse particles having a single pure crystalline phase of aRE-containing lattice, a uniform three-dimensional size, and a uniformpolyhedral morphology. In a reaction vessel the method first dissolves aprecursor metal salt, or a mixture of precursor metal salts, in asolvent to form a solution. The reaction vessel is then placed in aheated salt bath having a temperature of at least about 340° C. Applyingheat to the salt bath to rapidly decompose the precursor metal salts inthe solution to form the monodisperse particles. The reaction vessel isthen kept in the salt bath for a time sufficient to increase the size ofthe monodisperse particles. The reaction vessel is then removed from thesalt bath; and the reaction quenched with ambient temperature solvent.

The particles of the invention are synthesized through thermaldecomposition of precursor metal salts of the desired particlecomposition components. One of ordinary would know the precursor saltswhich may be used to yield a particular particle composition. Theprecursor metal salt may be a trifluoroacetate, chloride, nitrate,acetylacetonate, acetate, a diethyl-dithiocarbamate (ddtc) salt, a 1, 10phenanthroline diethyl-dithiocarbamate (phen)(ddtc) salt, an oleate, anoxalate, and the like. The halide compositions may be prepared fromthrihoaceates, e.g., trifluoracetates for fluoride composition. Theoxide composition may be prepared from acetates, actylacetoantes, etc.The oxysulfides are prepared from precursor salts containing oxygen andsulfur, e.g., (phn)(ddtc) salt. The sulfides may be prepared the same asthe oxysulfides but under hydrogen/argon atmosphere. The oxychloridesare prepared the same as the oxysulfides but using RE chlorideprecursors. The desired stoichiometric combination of precursor metalsalts are first dissolved in a solvent to form a solution of theprecursor compounds, for example in a 100 mL, 3-neck flat-bottom flask.Suitable solvents include, but are not limited to, a mixture of oleicacid and 1-octadecene, oleylamine, trioctylamine, trioctylphosphine,squalene, trioctylphosphine oxide, hexadecylamine, and the like, whichare typically solids at room temperature. A preferred solvent for thesynthesis of monodisperse particles of the invention is a mixture ofoleic acid and 1-octadecene. The oleic acid and 1-octadecene may bemixed in a 1:1 ratio. The mixture is typically heated under vacuum at100° C. to dissolve the trifluoracetate salts and remove excess water.The reaction vessel containing the hot is preferably purged with aninert gas such as nitrogen. The vessel is then placed in a molten saltbath while still purging with the inert gas. The salt bath should have atemperature of at least about 340° C. Salt baths known in the art may beused, with a 1:1 KNO₃/NaNO₃ salt bath being preferred. The salt bathacts as the heat reservoir to ensure the fast and uniform heating of thesolution and also to compensate the disparity in decompositiontemperature among various trifluoroacetate salts. The temperature of thesalt bath should be maintained throughout the entire reaction. Once thereaction is complete, the vessel is removed from the salt bath and thereaction quenched with room temperature solvent and the reactionstopped. In the case of a solvent mixture, such as oleic acid and1-octadecene, one or both components can be used—for oleicacid/1-octadecene, 1-octadecene may preferably be added to quench thereaction. The product particles may then be precipitated by addition ofan antisolvent, such as ethanol, and recovered by means known in the artsuch as centrifugation, filtration, etc. The particles may be washed byresuspending them in a non-polar solvent such as hexane, recovered, anddried at room temperature, with heat, and/or with vacuum.

When oleic acid/1-octadecene is used as the solvent, it has beenobserved using FTIR that some oleic acid attaches to the particlesurface during the reaction, although subsequent washings of theparticles can remove oleic acid. This is presumably due to thecarboxylic acid functional group in the oleic acid. Conducting thereaction in the presence of solvents having such functional groups whichmay attach to the surface of the monodisperse particles, or adding suchcompounds to the reaction, then is one route to introduce functionalityto the particle surface. In the case of oleic acid, although not to bebound by theory, it is believed that the oleic acid acts as asurfactant, assisting in the coordination of the precursor latticeframework. Under high temperature, oleic acid molecules form ionic bondsbetween the carboxyl group of the oleic acid and the RE ions in thecrystal lattice. The oleic acid functionalization also is believed toassist in suspending the monodisperse particles of the invention inhexane or other non-polar solvents and in their self-assembly intosuperlattices. Monodisperse particles of the invention having oleic acidsurface modification represent another embodiment of the invention.

Particle size and morphology may be controlled by varying reactionconditions such as stoichiometric precursor metal salt ratio, heatingrate of the salt bath, and reaction time. The initial rate of heating inthe salt bath is important in determining the morphology by selectingwhich crystal planes will undergo the most rapid growth. Final particlesize is determined by total reaction time in the salt bath as well asprecursor ratios. After the reaction vessel reaches the temperature ofthe salt bath, the longer the time the vessel remains in the salt baththe larger the particles may grow.

Superlattice Assembly

Due to their uniformity in size and morphology, the monodisperseparticles of the invention are able to self assemble into superlatticestructures. These superlattice structures represent the lowest freeenergy conformation for the assemblage. This uniform build-up isaccomplished with monodisperse particles of uniform size and morphologyas according to the invention. The superlattices form via interfacialself-assembly, building hierarchical structures with orders on differentlength scales. Superlattices of the monodisperse particles of theinvention may be formed by suspending the particles in a solvent andthen drop-casting them onto a surface. As the solvent slowly evaporates,the particles arrange themselves into a superlattice with bothpositional and orientational order. Any solvent which disperses theparticles may be used, such as, but not limited to, benzene, carbontetrachloride, chlorobenzene, chloroform, cyclohexane,dimethyl-formamide, dimethyl sulfoxide, ethanol, heptane, hexane,pentane, tetrahydrofuran, toluene, with non-polar organic solvents suchas hexane being preferred.

Superlattices of the invention may be transparent films of themonodisperse particles of the invention, particularly with monodispersenanoparticles of the invention. In order to form a superlattice theconstituent particles must be of identical or nearly identical size andshape. When both conditions are met a uniform, patterned, monolayer ofparticles forms. Advantageously, the monodisperse particles of theinvention meet this criteria for uniform size and uniform morphology.Due to the small size and uniformity of the particles of the invention,there is no scattering of light and as a result a transparent film isobtained.

Uses

With their uniform three-dimensional size and uniform polyhedralmorphology, the monodisperse particles of the invention are superior toinorganic phosphors known in the art. Just as with currently knowninorganic particles, monodisperse particles of the invention may be usedas labels or taggants for security/authentication uses, in assays, inbiomedical applications, etc., such as described above. In securityapplications, for example, the ability to tailor not only the particlecomposition but also its morphology gives infinite amount ofcombinations which can be loaded into inks, onto fibers, etc. fordocuments with unique signatures based on particles having differentmorphologies and photonic properties. Multi-component security fiberswhich can be incorporated into a document during the papermaking processcan have monodisperse particles of the invention with the samecomposition but different morphologies in the sheath and core of thefiber. They can be distinguished optically through their temporalproperties. The monodisperse particles or a combination of differentmonodisperse particles may be introduced into the fiber by absorption orby addition to the fiber stock in the fiber making process as is knownin the art. U.S. Pat. No. 6,974,623, for example, describes theincorporation of luminescent materials into mottled fibers for securitydocuments and is incorporated herein by reference. Using techniquesknown in the art, monodisperse particles may also be incorporated intoan ink, as a pigment, and the ink used to print at least a portion of asecurity document. Accordingly, a security document containingmonodisperse particles of the invention is an embodiment of theinvention. A fiber containing monodisperse particles of the invention isanother embodiment of the invention.

In biomedical applications, a particular phosphor composition may haveregulatory approval for its use and particles of the invention, withtheir different uniform morphologies, can provide a unique combinationof particles of the composition but with different optical/photonicproperties. In another embodiment of the invention, the monodisperseparticles may be functionalized by surface-treating or coating with abiological macromolecule which is adsorbed or chemically bonded to theparticle surface. Exemplary biological macromolecules may be, forexample, antibodies, streptavidin, avidin, proteins, a lipoprotein,polypeptide ligands of cellular receptors, polynucleotide probes, drugs,antigens, toxins, and the like. Surface-treating the monodisperseparticles of the invention with such biological macromolecules can beaccomplished using techniques known in the art (see, e.g., U.S. Pat.Nos. 5,043,265 and 5,698,397), and functionalizes the particles for usein assays, biomedical diagnostics and imaging, and medical treatmentsuch as photodynamic therapy. The oleic acid-functionalized monodisperseparticles of the invention may undergo a ligand exchange replacing thehydrophobic oleic acid molecules with a biological macromolecule asdescribed or with various ligands or polymers such as polyethyleneglycol, PEG; polyacrylic acid, PAA; polyethyleneimines, PEI; and otherligands and polymers known in the art. The ligands or polymers may behydrophilic or lipophilic. The functionalization coatings are adhered tothe crystal surface by physical adsorption and/or ionic interactionsbetween the RE ions and various carboxyl groups, amine groups, or otherfunctional groups. The carboxylic acid or amine derivatized particlescan be further functionalized with avidin, protein, antibody, or anyother biological or chemical agents through covalent linkage andactivation of surface amines or carboxyls. Accordingly, the inventionrelates to a monodisperse particle of the invention carrying abiological macromolecule bound or physically adsorbed to the particlesurface. The particle may be a monodisperse phosphor particle of theinvention which can function as a luminescent label. Another embodimentrelates to monodisperse particles have a ligand or a polymer bound orphysically adsorbed to its surface.

As discussed above, the monodisperse particles of the invention possessthe unique ability to self assemble into superlattices. In suchsuperlattices there are cooperative interactions which produce physicalproperties recognized as characteristic of bulk materials. Like atoms ormolecules, but in the next level of hierarchy, monodisperse particles ofthe invention, particularly in the form of nanocrystals, serve as thebuilding blocks to new designer solids. Techniques to produce wellordered 2D monolayers and 3D colloidal crystalline solids are alsofinding wide application. (See, e.g., C B Murray, C R Kagan and M GBawendi, Science 270 (5240):1335-1338, Nov. 24, 1995; D V Talapin, E VShevchenko, C B Murray, A Kornowski, S Forster and H Weller, JACS 126(40):12984-12988, Oct. 13, 2004). The assembly of nanocrystals canenable design of new solid state materials and devices.

As discussed herein, an interfacial assembly process may be used toorganize the monodisperse particles into superlattices over multiplelength scales (from nanometer to submillimeter scale). Thisadvantageously facilitates particle characterization and enablessystematic studies of shape-directed assembly. The global and localordering of these superstructures is programmed by the control overmonodisperse particle's anisotropy attributes such as faceting, aspectratio, etc., which are features of the monodisperse particles of theinvention.

The superlattices of the invention are nanocrystal thin films which canbe utilized in many different fields such as optical films forenhancement of solar cells, XRay scintillators, heat reduction throughblocking/reflecting of IR light. Other applications include lighting,detectors, displays, etc., solar and other energy efficient lightingsolutions such as LEDs and energy converting coatings for fluorescentlighting, which are high priorities for the nanophosphor technology. Insolar applications, the nanocrystals, as monodisperse particles orsuperlattices, can enhance the overall efficiency of the cell withoutgreatly increasing the cost. The ability to tune the absorption andemission properties of the monodisperse particles of the invention upand down the electromagnetic spectrum allows for the design of materialswhich absorb the atmospheric solar wavelengths outside the absorptionrange of conventional silicon-based solar cells and convert those tovisible and near IR wavelengths within the capacity of silicon. Themonodisperse particles and superlattices of the invention may beincorporated into various thin films for the solar cells, which providean excellent substrate for efficient energy conversion of the solarspectrum due to the minimal scattering and other losses that would beseen in less uniform nanocrystals.

NaYF₄-Based Particles as an Illustration of the Invention

By way of example to illustrate the monodisperse particles of theinvention, but not to limit the invention, the following discussion isdirected to NaYF₄-based UCNPs, which represent a preferred embodiment ofthe invention.

Hexagonal phase, β-NaYF₄-based UCNPs were synthesized through thermaldecomposition of sodium and lanthanide trifluoroacetates dissolved in amixture of oleic acid and 1-octadecene. The use of molten salt bath asthe heat reservoir ensures the fast and uniform heating of the solutionand also to compensate the disparity in decomposition temperature amongvarious trifluoroacetate salts. Transmission electron microscopy (TEM)images of the UCNPs of various shapes and compositions are shown in FIG.3. FIG. 3 shows TEM images of the β-NaYF₄-based UCNPs; (a, d, g, j) TEMimages of NaYF₄:Yb/Er (20/2 mol %) UCNPs; (b, e, h, k) TEM images ofNaYF₄:Yb/Tm (22/0.2 mol %) UCNPs; (f, i) TEM images of NaYF₄:Yb/Ho (20/2mol %) UCNPs; and (c, l) TEM images of NaYF₄:Yb/Ce/Ho (20/11/2 mol %)UCNPs. All scale bars in FIG. 3 represent 100 nm. For the case ofNaYF₄:Yb/Er (20/2 mol %), an optimized composition for efficientupconversion (32), the morphologies of the UCNPs can be tuned fromspherical hexagon nanocrystals (FIG. 3a ), nanorods (FIG. 3d ),hexagonal nanoprisms (FIG. 3g ) to hexagonal nanoplates (FIG. 3j ) byadjusting the reaction time or the ratio of sodium to lanthanidetrifluoroacetates. The β-NaYF₄ monodisperse particles of the inventioncan take the form of hexagonal spheres, prisms, or plates as shown inFIG. 3. For each of the morphologies possess a pure hexagonal crystalphase. The particles on FIG. 3a appear spherical due to their small sizeand the limits of image resolution, but in fact are hexagonal and may belabeled “spherical hexagons.”

The structural and optical characterization of the β-NaYF₄-based UCNPsare shown in FIG. 4: (a) Powder X-ray diffraction patterns of theNaYF₄:Yb/Er (20/2 mol %) UCNPs with different shapes. The peaks areindexed according to the standard pattern of β-NaYF₄(JCPDS file number:28-1192). Insets are the corresponding structural models. (b)High-resolution TEM (HRTEM) image of a spherical NaYF₄:Yb/Er (20/2 mol%) UCNP. (c) HRTEM image of a NaYF₄:Yb/Er (20/2 mol %) nanorod. (d)HRTEM image of a NaYF₄:Yb/Er (20/2 mol %) hexagonal nanoprism. (e) HRTEMimage taken from the edge of a NaYF₄:Yb/Er (20/2 mol %) hexagonalnanoplate. (f) Room temperature upconversion emission spectra of theNaYF₄:Yb/Er (20/2 mol %) and NaYF₄:Yb/Tm (22/0.2 mol %) UCNPs dispersedin hexane. Inset: Photographs of the upconversion luminescence from theNaYF₄:Yb/Er (20/2 mol %) (left) and NaYF₄:Yb/Tm (22/0.2 mol %) (right)nanorod solutions under 980 nm diode laser excitation. (g) Roomtemperature upconversion emission spectra of the NaYF₄:Yb/Ho (20/2 mol%) and NaYF₄:Yb/Ho (20/1 mol %) UCNPs dispersed in hexane. Inset:Photographs of the upconversion luminescence from the NaYF₄:Yb/Ho (20/2mol %) nanoprism (left), NaYF₄:Yb/Ho (20/2 mol %) nanorod (middle) andNaYF₄:Yb/Ho (20/1 mol %) nanorod (right) solutions under 980 nm diodelaser excitation. Powder X-ray diffraction (XRD) patterns confirm thatall the NaYF₄:Yb/Er (20/2 mol %) UCNPs are of a single pure β-NaYF₄crystalline phase (FIG. 4a ). The XRD patterns of the spherical NCs andthe nanorods exhibit enhanced (h00) as well as diminished (002)reflections, whereas a reversed trend is observed in the case ofhexagonal nanoprisms and nanoplates. These results imply that themajority of spherical hexagon nanocrystals and nanorods are as-depositedwith the [0001] direction (c-axis) parallel to the glass substrates usedfor XRD measurements. In contrast, most hexagonal nanoprisms andnanoplates are sitting with the c-axis perpendicular to the substrates.Supporting this interpretation are the results of TEM characterizationof samples prepared in a similar way (fast deposition from a hexanesolution). High-resolution TEM (HRTEM) image of a single spherical NCshows clear lattice fringes associated with the (10 0), (10 1) and(0001) hexagonal crystal planes, respectively (FIG. 4b ). Latticefringes corresponding to the (0001) planes appear along the nanorods,indicating that the nanorods grow along the c-axis (FIG. 4c ). HRTEManalysis also reveals that the “cube-like” hexagonal prism is composedof six square or rectangular {10 0} side facets with two hexagonal basesbelonging to the (0001) planes (FIG. 4d ). The formation of nanorods andhexagonal nanoprisms is determined by the delicate interplay between thegrowth rates of {0001} and {10 0} planes at different growth stages.This is in contrast to the previous studies where shape evolution of theβ-NaYF₄ nanocrystals was dominated by the Ostwald-ripening process (31).Furthermore, dynamic light scattering experiments that probe thehydrodynamic size of the dispersed nanorods and hexagonal nanoprismsshow results consistent with the largest dimensions of individualnanocrystals measured from the TEM images. In addition, quantitativeelemental analyses based on inductively coupled plasma optical emissionspectrometry (ICP-OES) indicate the proportional incorporation ofprecursor lanthanide elements into the final UCNPs. By increasing thesodium to lanthanide ratio and the reaction time, hexagonal nanoplateswith an edge length of 133±5 nm and a thickness of 104±8 nm are obtained(FIG. 3j ). HRTEM image taken from the edge region confirms its highcrystallinity (FIG. 4e ). It is worth pointing out that although thepossibility of cubic to hexagonal (α→β) phase transition cannot be ruledout under the present synthesis conditions, no definitive signature ofthis was observed.

The NaYF₄:Yb/Er (20/2 mol %) UCNPs of the invention exhibit intenseupconversion luminescence under 980 nm excitation (FIG. 4f ). Threevisible emission bands centered at 525, 542, and 655 nm are observed,attributable to the radiative transitions from the (²H_(11/2), ⁴S_(3/2))(green) and from the ⁴F_(9/2) (red) excited states to the ⁴I_(15/2)ground state of Er³⁺, respectively. The activator Yb³⁺, capable ofabsorbing the 980 nm near-infrared light efficiently, transfers itsenergy sequentially to nearby Er³⁺ through the ²F_(5/2) (Yb³⁺) 4I_(11/2)(Er³⁺) process, pumping the Er³⁺ to its emitting levels. The multiphononrelaxation processes help bridging the different excited states of Er³⁺,giving rise to distinct emission peaks. The NaYF₄:Yb/Er (20/2 mol %)UCNPs of the invention obtained, display size-dependent opticalproperties (FIG. 4f ). Both the total intensity of emission and theintensity ratio of green to red emission increase as the NCs get larger.This can be ascribed to the fact that as the size of the nanocrystalsdecreases, surface defects- and ligands-induced quenching ofupconversion become more important, which modifies the relativepopulation among various excited states through phonon-assistednonradiative relaxations (32). In addition, larger nanocrystals containmore emission centers (Er³⁺) under the same doping level and thus, areexpected to be brighter. Therefore, engineering not only the dopantconcentration but also the surface functionalities of the UCNPs can bean effective means of tuning the upconversion luminescence.

To demonstrate the generality of the synthesis method and further tailorthe upconversion emissions, several other exemplary dopant combinationswere prepared including Yb/Tm, Yb/Ho, and Yb/Ho/Ce for the β-NaYF₄-basedUCNPs. TEM images of the NaYF₄:Yb/Tm (22/0.2 mol %) UCNPs with differentmorphologies are shown in FIG. 3b (spherical hexagonal nanocrystals),FIG. 3e (nanorods), FIG. 3h (hexagonal nanoprisms), and FIG. 3k(hexagonal nanoplates), respectively. Upon 980 nm excitation, thesehexagonal phase UCNPs emit bright blue upconversion luminescence arisingfrom the trivalent thulium ¹D₂→³F₄ and ¹G₄→³H₆ transitions (FIG. 4f ).In addition, predominantly green upconversion emissions were observedfrom the hexagonal phase NaYF₄:Yb/Ho (20/2 mol %) nanorods andnanoprisms UCNPs (FIG. 3f, 3i, 4g ). The intensity ratio of green to redemission from the nanorods increases as the Ho³⁺ concentration increasesfrom 1% to 2%, owing to the enhanced energy transfer from the Yb³⁺sensitizers to adjacent Ho³⁺ ions (33). Furthermore, we have employedthe Ce³⁺ ions to modulate the upconversion profiles of the NaYF₄:Yb/Ho(20/2 mol %) UCNPs. The parity-allowed 4→5d transition in the Ce³⁺ ionscan effectively depopulate the green-emitting ⁵F₄/⁵S₂ states of Ho³⁺while increasing the population of the red-emitting ⁵F₅ state of Ho³⁺through two cross relaxation pathways:⁵F₄/⁵S₂(Ho³⁺)+²F_(5/2)(Ce³⁺)→⁵F₅(Ho³⁺)+²F_(7/2)(Ce³⁺) and⁵I₆(Ho³⁺)+²F_(5/2)(Ce³⁺)→⁵I₇(Ho³⁺)+²F_(7/2)(Ce³⁺) (34). Theas-synthesized NaYF₄:Yb/Ce/Ho (20/11/2 mol %) spherical hexagonalnanocrystals (FIG. 3c ) and hexagonal nanoplates (FIG. 3I) displaypredominantly red emission under 980 nm excitation although the totalintensity of emission is much weaker than other UCNPs. Powder XRDpatterns confirmed that both samples are of pure hexagonal phase. Thesystematic peak shifts to lower angles compared to the standard XRDpattern of β-NaYF₄ imply the partial substitution of Y³⁺ ions by thelarger Ce³⁺ ions in the β-NaYF₄ lattice (Y³⁺, r=1.159 Å; Ce³⁺, r=1.283Å) (35), which results in the expansion of the unit cell. This differsfrom previous observation where dopant with a large ionic radii couldnot be incorporated into the β-NaYF₄ lattice (20). In addition, uniformundoped β-NaYF₄ nanorods can also be synthesized and represent anembodiment of the invention.

Superlattices composed of anisotropic nanocrystals have attracted greatinterest due to the rich phase behaviors and the emergent collectiveproperties (36). Further exploiting the intriguing structural diversityof ordered nanocrystal assemblies using β-NaYF₄ nanorods and theβ-NaYF₄-based UCNPs superlattices were formed. FIG. 5 shows NaYF₄(AR=1.4) nanorod superlattices. (a) TEM image of a monolayersuperlattice of nanorods that are oriented parallel to the substrate.The upper right inset is the corresponding selected-area wide-angleelectron diffraction pattern (SAWED) and the lower right inset is thecorresponding small-angle electron diffraction pattern (SAED). Bothpatterns are acquired from an area of ˜6.5 μm². (b) TEM image of adouble-layer superlattice of nanorods that are oriented parallel to thesubstrate. The upper left inset is the high-magnification TEM imageacquired from the same domain. The upper right inset is thecorresponding SAWED pattern and the lower right inset is thecorresponding SAED pattern. Both patterns are acquired from an area of˜6.5 μm². (c) Optical micrographs of the NaYF₄(AR=1.4) nanorodsuperlattices observed with crossed polarizers. The scale bar represents30 μm. The birefringence is due to the alignment of the nanorods. (d) AnAFM image of the nanorod assemblies showing the domain boundaries.

An interfacial assembly strategy that can produce continuous and uniformnanocrystal superlattice films (37) was employed. When 15 μL of hexanesuspension of the β-NaYF₄ nanorods with an aspect ratio (AR) of ˜1.4 isdrop-cast onto the viscous and weakly polar ethylene glycol (EG) surfaceand is allowed for slow solvent evaporation, large area nanorodsuperlattices comprised of monolayer and double-layer domains areobtained (FIG. 5a, 5b ). The nanorods preferentially aligned with theirc-axis parallel to the substrate, exhibiting both positional andorientation order, as evidenced by the corresponding SAED patterns. Thestriking in-plane ordering of the nanorod superlattices are alsorevealed by the SAWED patterns, whose spots are due to diffraction ofcrystallographic lattice planes. Specifically, the strong (002)diffraction spots arise from the anisotropic rod-shape of the individualNCs and their mutual alignment along the c-axis. Interestingly, theappearance of (h00) and simultaneous absence of (110) diffraction spots,together with the recognition that the β-NaYF₄ nanorods are enclosed bythe {10 0} facets, allows us to conclude that the nanorods areazimuthally aligned along their {10 0} crystal faces. This is accountedfor by the in-plane, dense packing of the β-NaYF₄ nanorods, possessing ahexagonal cross section and also the interdigitation of ligands,contributing to attractions between adjacent nanorods. Further evidencethat supports the explanation is the lateral displacement betweenneighboring layers in the β-NaYF₄ nanorods superlattices. Liquidcrystalline order has been observed in concentrated nanorod dispersions(38) and nanorod films prepared by controlled evaporation (39, 40) or byLangmuir Blodgett assembly (41). Viewing the superlattices through apolarizing optical microscope showed the ordering of nanorod films.Optical micrographs (FIG. 5c ) indicate domains that are stronglybirefringent due to the alignment of nanorods. The multi-domain natureof the nanorod films is also confirmed by the atomic force microscopy(AFM) study. A nanorod with a larger AR (>3) would be better suited forthe formation of liquid crystalline phases.

FIG. 6 shows NaYF₄ (AR=2.0) nanorod superlattices. (a) TEM image of amonolayer superlattice of nanorods that are oriented parallel to thesubstrate. The lower right inset is the corresponding SAED patternacquired from an area of ˜2 μm². (b) TEM image of a double-layersuperlattice of nanorods that are oriented parallel to the substrate.The lower right inset is the corresponding SAED pattern acquired from anarea of ˜2 μm². (c) TEM image of a monolayer of vertically alignednanorod superlattices. The upper left inset is the high-magnificationTEM image showing the hexagonally closed-packed array of nanorods. Theupper right inset is the corresponding SAWED pattern acquired from anarea of ˜60 μm². (d) TEM image of a closed-packed hexagonally orderedarray of vertically aligned nanorods. The upper right inset is thecorresponding SAWED pattern and the lower right inset is thecorresponding SAED pattern. Both patterns are acquired from an area of˜6.5 μm.

β-NaYF₄ nanorods with an AR of ˜2.0 were also used to study theshape-directed assembly behavior: monolayer and double layersuperlattices are also obtained by depositing 15 μL of nanoroddispersion (FIG. 6a, 6b ). However, when 40 μL of nanorod solution isused, an extended domain (˜200 μm²) of vertically aligned nanorods isobtained (FIG. 6c ), as seen from the corresponding SAWED pattern. Thedomain is composed of hexagonally closed-packed perpendicularly alignednanorod superlattices (FIG. 6d ). This is the preferred geometry ofnanorod arrays for various applications such as photovoltaics (42).

FIG. 7 shows hexagonal nanoprism and nanoplate superlattices. (a) SEMimage of a monolayer superlattice of NaYF₄:Yb/Tm (22/0.2 mol %)hexagonal nanoprisms. The upper right and lower right insets are thehigh-magnification SEM and TEM images, respectively, showing the localpacking motif of the superlattice. (b) SEM image of the self-assembledsuperlattice of NaYF₄:Yb/Er (20/2 mol %) hexagonal nanoplates. The upperright and lower right insets are the high-magnification SEM and TEMimages, respectively, showing the hexagonally closed-packed array of thenanoplates.

The ordering of the as-deposited hexagonal nanoprisms and nanoplatesfilms is also strongly dependent on the detailed geometry of individualNCs: Close inspection indicates that in the hexagonal nanoprismassemblies (FIG. 7a ), each “cube-like” nanoprism has six neighbors andthe packing symmetry deviates from the square lattice that are expectedfor perfect cubes. The arrangement, in light of recent theoretical workon the packing of fourfold rotationally symmetric superdisks, can beassigned to the Λ₁ lattice packing. Due to the reduced shape symmetry,the hexagonal nanoprisms self-organize into a configuration thatmaximizes the packing density. On the other hand, hexagonal nanoplatesself assemble into closed-packed hexagonally ordered arrays (FIG. 7b ),consistent with the six-fold symmetry of nanoplates.

EXAMPLES

Materials and Methods

Dehydration of Solvents

Precursor salts (trifluoroacetates, chlorides, nitrates,acetylacetonate, acetate, RE(phen)(ddtc)₃) ratios are combined asindicated in each example in a 100 ml, 3-neck round bottom flask anddissolved in 1:1 ratio of 1-octadecene/oleic acid (ODE/OA) solvents byheating at ˜110° C., under vacuum for at least 45 min using a heatingmantle. After 45 min, water is removed from the RE precursor solution;the flask is purged for ˜5 min with research grade, nitrogen gas tocreate an inert atmosphere.

Salt Bath Preparation

The KNO₃/NaNO₃ salt bath was prepared by combining a 1:1 ratio by weightin a 30 cm in diameter glass heating bath. The salt bath is heated to asteady state temperature of at least 340° C. using a stirring hot plate.

Example 1: Synthesis of Upconversion Nanophosphors (UCNPs)

All syntheses were carried out using standard Schlenk techniques andcommercially available reagents. 1-Octadecene (ODE; technical grade,90%), oleic acid (OA; technical grade, 90%), Na(CF₃COO), and ethyleneglycol (EG) were purchased from Sigma Aldrich. RE(CF₃COO)₃ (RE=Y, Yb,Er, Tm) and Y, Yb, and Er 1,000 ppm ICP standard solutions werepurchased from GFS Chemicals, Inc. Ho(CF₃COO)₃ was purchased from RareEarth Products, Inc. Ce(CF₃COO)₃ was prepared according to theliterature method using cerium(III) carbonate hydrate (Aldrich) andtrifluoroacetic acid (Alfa Aesar) as the precursors. A typical protocolfor the synthesis of hexagonal phase NaYF₄-based UCNPs is describedbelow: certain amount of Na(CF₃COO) and Re(CF₃COO)₃ together with 15 mLof ODE and 15 mL of OA were added to a three-necked flask. The mixturewas then heated under vacuum at 100° C. for 45 min to form atransparent, light-yellow solution. The reaction flask was flushed withN₂ for 5 min and was then placed into a molten NaNO₃/KNO₃ (1:1 massratio) salt bath that was pre-stabilized at 342° C. A large amount ofwhite smoke was produced after about 1.5 min, indicating thedecomposition of metal trifluoroacetates. After 20-35 min of reactionunder N₂ flow and vigorous magnetic stirring, the solution was cooleddown by adding 15 mL of ODE. The products were isolated by addingethanol and centrifugation. No size-selective fractionation is needed.The UCNPs were redispersed in hexane with nanocrystal concentration ofabout 5.0 mg/mL. The synthetic conditions for exemplary NaYF₄-basedparticles are given in Table 1. The RE trifluoroacetates RE(CF₃COO)₃(RE=Y, Yb, Er, Tm, Ce, Ho) used were obtained as trihydrates (i.e.,RE(CF₃COO)₃·3H₂O) and Na(CF₃COO) was anhydrous form.

TABLE 1 Synthesis Conditions of β-NaYF₄-based UCNPs with 342° C. saltbath. Composition Na(CF₃COO)₃ Y(CF₃COO)₃ Dopants Time MorphologyNaYF₄:Yb,Er 6.18 mmol 2.60 mmol 0.68 mmol Yb(CF₃COO)₃ 21 min SphericalHexagon 0.068 mmol Er(CF₃COO)₃ NaYF₄:Yb,Er 5.62 mmol 2.60 mmol 0.68 mmolYb(CF₃COO)₃ 23 min Rod 0.068 mmol Er(CF₃COO)₃ NaYF₄:Yb,Er 5.62 mmol 2.60mmol 0.68 mmol Yb(CF₃COO)₃ 28 min Hexagonal Prism 0.068 mmol Er(CF₃COO)₃NaYF₄:Yb,Er 6.37 mmol 2.60 mmol 0.68 mmol Yb(CF₃COO)₃ 33 min HexagonalPlate 0.068 mmol Er(CF₃COO)₃ NaYF₄:Yb,Tm 5.90 mmol 2.60 mmol 0.72 mmolYb(CF₃COO)₃ 21 min Spherical Hexagon 0.0072 mmol Tm(CF₃COO)₃ NaYF₄:Yb,Tm5.62 mmol 2.60 mmol 0.72 mmol Yb(CF₃COO)₃ 23 min Rod 0.0072 mmolTm(CF₃COO)₃ NaYF₄:Yb,Tm 5.62 mmol 2.60 mmol 0.72 mmol Yb(CF₃COO)₃ 28 minHexagonal Prism 0.0072 mmol Tm(CF₃COO)₃ NaYF₄:Yb,Tm 6.37 mmol 2.60 mmol0.72 mmol Yb(CF₃COO)₃ 33 min Hexagonal Plate 0.0072 mmol Tm(CF₃COO)₃NaYF₄:Yb,Ho 5.62 mmol 2.60 mmol 0.68 mmol Yb(CF₃COO)₃ 23 min Rod 0.068mmol Ho(CF₃COO)₃ NaYF₄:Yb,Ho 5.62 mmol 2.60 mmol 0.68 mmol Yb(CF₃COO)₃28 min Hexagonal Prism 0.068 mmol Ho(CF₃COO)₃ NaYF₄:Yb,Ho,Ce 5.62 mmol2.25 mmol 0.68 mmol Yb(CF₃COO)₃ 22 min Spherical Hexagon 0.068 mmolHo(CF₃COO)₃ 0.36 mmol Ce(CF₃COO)₃ NaYF₄:Yb,Ho,Ce 5.62 mmol 2.25 mmol0.68 mmol Yb(CF₃COO)₃ 28 min Hexagonal Plate 0.068 mmol Ho(CF₃COO)₃ 0.36mmol Ce(CF₃COO)₃ NaYF₄ 4.22 mmol 2.23 mmol None 20-22 min   Rod

Example 2: Superlattice Formation

2.1 Assembly of UCNPs into Superlattices.

A 1.5×1.5×1 cm³ Teflon well was half-filled with ethylene glycol (EG).Varying concentrations of UCNPs from Example 1, ranging from 1 mg/ml to20 mg/ml are prepared in hexane, 15 μl of suspension was drop-cast ontothe EG surface and the well was then covered by a glass slide to slowdown solvent evaporation. This differentiates our assembly setup frombeing a miniature Langmuir-Blodgett trough without mechanical barriers.After 40 min, the nanocrystal film was transferred onto glass substratesor TEM grids (300-mesh) that was further dried under vacuum to removeextra EG.

2.2 Structural and Optical Characterization

Transmission electron microscopy (TEM) images and electron diffractionpatterns were taken on a JEM-1400 Microscope operating at 120 kV.High-resolution TEM (HRTEM) images were taken on a JEOL2010F microscopeoperating at 200 kV. Scanning electron microscopy (SEM) was performed ona JEOL 7500F HRSEM. Power X-ray diffraction (XRD) patterns were obtainedon the Rigaku Smartlab diffractometer at a scanning rate of 0.1° min⁻¹in the 2θ range from 10° to 80° (Cu Kα radiation, λ=1.5418 Å). For XRDmeasurement, samples were prepared by depositing hexane solutions ofnanocrystals onto a glass substrate. Dynamic light scattering (DLS)experiments were performed on a Delsa Nano C system (Beckman Coulter).Atomic Force Microscope (AFM) height images were obtained on the DIMultimode AFM. Quantitative elemental analysis was carried out withinductively coupled plasma optical emission spectrometry (ICP-OES) on aSPECTRO GENESIS ICP spectrometer. Room temperature upconversion emissionspectra were acquired with the fiber-optically coupled USB4000fluorescence spectrometer (Ocean Optics) using an external 200 mWcontinuous-wave laser centered at 980 nm as the excitation source(Dragon Lasers). The optical photographs of the emitting UCNPs weretaken using a Nikon D300 digital camera. Nanorod superlattices on glasssubstrates were imaged under crossed polarizers using a Leica DMRXupright microscope equipped with a charge-coupled device (CCD) camera(Hitachi KP-M1U).

Example 3: Synthesis of NaYF₄:Yb,Er and LiYF₄:Yb,Er Particles of theInvention

The reaction flask containing the dissolved precursors is submerged intothe molten salt bath while still purging with N₂ at a steady flow rate.It is important that the salt bath maintains a steady temperature rangeof 340-343° C. for the entirety of the reaction. After completion of thereaction, the flask is removed from the salt bath and immediatelyquenched with room temperature ODE. The product is precipitated withethanol and centrifuged to obtain the nanoparticles. Fourier TransformInfraRed spectroscopy (FTIR) showed oleic acid to be present on theparticles' surfaces.

The above protocol is the standard protocol for synthesis of halide,sulfide, and oxysulfide nanoparticles. Particle size and morphology canbe controlled by varying reaction conditions such as precursor ratio,heating rate of the salt bath, and reaction time. The initial rate ofheating determines the morphology by selecting which crystal planes willundergo the most rapid growth. Final particle size is determined bytotal reaction time as well as precursor ratios.

Below is a description of the reaction conditions for three uniquemorphologies of both NaYF₄:Yb,Er and LiYF₄:Yb,Er while maintainingnarrow size distributions and the same final chemical composition. FIG.8 shows TEM images of the unique morphologies in the NaYF₄:Yb,Ernanoparticle system.

TABLE 2 Reaction conditions for growth of sphere, rod, or prism NaYF₄beta-phase, RE-doped nanoparticles. Composition Na(CF₃COO) Y(CF₃COO)₃Dopants Time Morphology NaYF₄:Yb,Er 5.62 mmol 2.60 mmol  0.68 mmolYb(CF₃COO)₃ 21 min Spherical Hexagon 0.068 mmol Er(CF₃COO)₃ NaYF₄:Yb,Er5.62 mmol 2.60 mmol  0.68 mmol Yb(CF₃COO)₃ 23 min Rod 0.068 mmolEr(CF₃COO)₃ NaYF₄:Yb,Er 5.62 mmol 2.60 mmol  0.68 mmol Yb(CF₃COO)₃ 28min Hexagonal Prism 0.068 mmol Er(CF₃COO)₃

The NaYF₄:Yb,Er spherical hexagons and hexagonal prisms are acquired byrapid heating of the reaction flask within the initial nucleation phaseof the reaction. This allows for isotropic growth of each crystal plane.The reaction times will determine sphere or prism morphologies, shorterreaction time will provide spheres and longer reaction times will yieldlarger prisms. A decreased rate of reaction will yield anisotropicgrowth in the case of the rod shaped nanoparticles. Aspect ratio of thenanorods can also be controlled by reaction time. Table 3 is thequantitative measurement of the particle composition for NaYF₄:Yb,Ernanorods and hexagonal prisms. This data shows that unique morphologiescan be obtained while having identical or near identical compositionswhen compared by elemental analysis.

TABLE 3 Quantitative analysis of NaYF₄:Yb,Er rod and prism shapednanoparticles. Morphology Y/% Yb/% Er/% Rod 75.0 23.0 2.0 HexagonalPrism 77.6 20.4 2.0

TABLE 4 Reaction conditions for growth of sphere, diamond, orbi-pyramidal, LiYF₄ beta-phase, RE-doped nanoparticles. CompositionLi(CF₃COO)₃ Y(CF₃COO)₃ Dopants Temp Time Morphology LiYF₄:Yb,Er 5.62mmol 2.60 mmol  0.68 mmol Yb(CF₃COO)₃ 330° C. 22 min Sphere 0.068 mmolEr(CF₃COO)₃ LiYF₄:Yb,Er 5.62 mmol 2.60 mmol  0.68 mmol Yb(CF₃COO)₃ 343°C. 22 min Diamond 0.068 mmol Er(CF₃COO)₃ LiYF₄:Yb,Er 5.62 mmol 2.60 mmol 0.68 mmol Yb(CF₃COO)₃ 343° C. 28 min Tetragonal Bi- 0.068 mmolEr(CF₃COO)₃ Pyramidal

As can be seen in Table 4 for LiYF₄:Yb,Er, three unique morphologies canbe achieved through control of reaction temperature and time whilekeeping the starting precursor ratios the same. Images of these uniquemorphologies are shown in FIG. 9. FIG. 9a is a TEM image showing thespheres. FIG. 9b is a TEM image of the diamonds. FIG. 9c is an SEM imageshowing the tetragonal bipyramids. FIG. 10 shows the TEM images oftetragonal by pyramids in the top panel (FIG. 10A) at ˜150 nm and of thediamonds in the bottom panel (FIG. 10B) at ˜75 nm. For instance, toachieve either spheres or diamond nanoparticles a reduced reactiontemperature of ˜10-15° C. is required to yield spheres while a reactiontemperature of 343° C. will yield diamonds but the reaction times areidentical and both reactions will yield beta-phase LiYF₄:Yb,Ernanoparticles. The LiYF₄:Yb,Er spheres are an exception to thepolyhedral morphology of the monodisperse particles of the invention andrepresent a separate embodiment of the invention. Finally, a reactiontemperature of 343° C. and a longer reaction time of 28 min will yieldlarge (˜150 nm) bi-pyramidal nanoparticle morphologies.

Example 4: Functionalization of Monodisperse Particle

The following procedure describes the functionalization of the NaYF₄,oleic acid coated particles obtained by ligand exchange of the oleicacid. EDC (1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride,5 mg) and sulfo N-hydroxysuccinimide (sulfo-NHS, 15 mg) are added to MESbuffer (0.1 M, 20 mL, pH 6.0) containing ligand exchanged, PAA-coatedNaYF₄:Yb,Er nanoparticles (5 mg). The mixture is stirred for 8 h at roomtemperature. After centrifugation and washing with water, theprecipitate is added into PBS buffer solution (0.1 M, 5 mL, pH 7.2)containing streptavidin (2 mg). The linkage reaction is allowed toproceed at 4° C. for 48 h. Then lysine (20 mg) is added to remove anyunreacted sulfo-NHS. Finally, streptavidin-functionalized NaYF₄:Yb,Ernanoparticles are obtained by centrifugation, and washed three timeswith water.

Example 5: Exemplary Preparation of Other Monodisperse Particles of theInvention

The monodisperse particles of the invention listed in Table 5 wereprepared according to the exemplary preparations described below.

TABLE 5 Upconversion, Reaction Time Downconversion, CompositionMorphology (min) Other LaF₃, PrF₃, NdF₃, Circular plate 30-60Downconversion SmF₃ EuF₃ Orthorhombic 45 Downconversion EuF₃ (No LiTFA)Hexagonal 45 Downconversion TbF₃ Rhombohedral 30 Downconversion PlateDyF₃ Rhombohedral 30 Downconversion Plate HoF₃ Rhombohedral 30Downconversion Plate LiHoF₄ Tetragonal bi- 30 Downconversion pyramidLiErF₄ Tetragonal bi- 22 Downconversion pyramid Gd₂O₂S:Yb,Er Hexagonal45-60 Upconversion Plates GdF₃ Rhombic 30 Downconversion nanoplatesMagnetic Y₂O₃:Yb,Er Spherical 30-60 Upconversion Hexagons CeO₂ CircularPlates 30 Downconversion Oxygen Storage NaGdF₄:Tb Spherical 45Downconversion hexagons

5.1 Preparation of EuF₃

Monodisperse Particles of the Invention (Example of LnF₃ Synthesis whereLn can be any RE, in this case Ln is Eu.)

5.05 mmol of Li trifluoroacetate (LiTFA) and 3.26 mmol of Eutrifluoroacetate are weighed and dissolved in a 1:1 mixture of ODE andOA in a 100 ml, 3-neck flask. The mixture is heated at 110° C. undervacuum for 45-60 min until a clear solution is obtained. The solution isthen transferred to a molten salt bath, maintained at a steadytemperature of 341-343° C. for the entirety of the reaction whilepurging with N₂ gas. The solution reacts for 45 min while stirring. Uponcompletion of the 45 min reaction, the flask is removed from the saltbath and the solution quenched with room temperature ODE.

The particles are precipitated with ethanol and centrifuged at 8300 rpmfor 3 min. The collected particles are washed once more with ethanol andre-suspended in hexane.

In the instance of EuF₃, two distinct morphologies were obtaineddepending on the presence of LiTFA. The single crystalline phase EuF₃will form a hexagonal nanoplate morphology, however, with the additionof LiTFA, the LiTFA will act as a mineralizer limiting the EuF₃hexagonal growth yielding an orthorhombic nanoplate. FIG. 11 is a TEMimage of the EuF₃ orthorhombic nanoplates. The scale bar is 100 nm. FIG.12 shows the visible red emission (˜610 nm) and associated absorptionand emission spectra of EuF₃ orthorhombic nanoplates under UVexcitation.

5.2 Preparation of Gd₂O₂S:Yb,Er

Monodisperse Particles of the Invention

2.6 mmol of Gd(phen)(ddtc)₃, 0.68 mmol of Yb(phen)(ddtc)₃, and 0.68 mmolof Er(phen)(ddtc)₃ were weighed and dissolved in a 1:1 ratio of ODE andOA in a 100 ml, 3-neck flask. The mixture is heated at 110° C. undervacuum for 45-60 min until a clear solution is obtained. The solution isthen transferred to a molten salt bath, maintained at a steadytemperature of 341-343° C. for the entirety of the reaction whilepurging with N₂ gas. The solution reacts for 45 min while stirring. Uponcompletion of the 45 min reaction, the flask is removed from the saltbath and the solution quenched with room temperature ODE.

The particles are precipitated with a hexane/acetone solution (1:1) andcentrifuged at 8300 rpm for 3 min. The collected particles are washedonce more with hexane/acetone and re-suspended in water.

5.3 Preparation of Y₂O₃:Yb,Er

Monodisperse Particles of the Invention

2.6 mmol of Yttrium acetate, 0.68 mmol of ytterbium acetate, and 0.068mmol of erbium acetate are weighed and dissolved in a 1:1 ratio of ODEand OA in a 100 ml, 3-neck flask. The mixture is heated at 110° C. undervacuum for 45-60 min until a clear solution is obtained. The solution isthen transferred to a molten salt bath, maintained at a steadytemperature of 341-343° C. for the entirety of the reaction whilepurging with N₂ gas. The solution reacts for 45 min while stirring. Uponcompletion of the 45 min reaction, the flask is removed from the saltbath and the solution quenched with room temperature ODE.

The particles are precipitated with a hexane/acetone solution (1:1) andcentrifuged at 8300 rpm for 3 min. The collected particles are washedonce more with hexane/acetone and re-suspended in water.

5.4 Preparation of CeO₂

Monodisperse Particles of the Invention

0.1 g cerium acetate hydrate, 0.53 g sodium diphosphate, 1 mL oleicacid, 2.5 mL oleylamine, and 4.5 mL 1-octadecene are used as startingmaterials. The mixture is heated to 120° C. while stirring for 20 minunder N₂ atmosphere. The mixture is then heated to 330° C. with vigorousmagnetic stirring and maintained at this temperature for 30 min under N₂atmosphere. After 30 min, the solution is cooled down and the ceriananoplates are flocculated by adding ethanol and centrifugation. Thenanoplates are redispersed in hexane.

5.5 Preparation of NaGdF₄:Tb

Monodisperse Particles of the Invention

2.6 mmol of Gadolinium trifluoroacetate and 0.68 mmol of terbiumtrifluoroacetate are weighed and dissolved in a 1:1 ratio of ODE and OAin a 100 ml, 3-neck flask. The mixture is heated at 110° C. under vacuumfor until a clear solution is obtained. The solution is then transferredto a molten salt bath, maintained at a steady temperature of 341-343° C.for the entirety of the reaction while purging with N₂ gas. The solutionreacts for 45 min while stirring. Upon completion of the 45 minreaction, the flask is removed from the salt bath and the solutionquenched with room temperature ODE.

The particles are precipitated with a hexane/acetone solution (1:1) andcentrifuged at 8300 rpm for 3 min. The collected particles are washedonce more with hexane/acetone and re-suspended in water. FIG. 13 is aTEM image showing the spherical hexagon morphology of NaGdF₄:Tbmonodisperse particles of the invention.

FIG. 14 is the emission spectra from NaGdF₄:Tb doped nanoparticles under6 megavolt X-Ray radiation. The overlaid spectrum is the absorption of aprotoporphyrin photosensitizer utilized for Photodynamic Therapy. TheNaGdF₄:Tb particles will absorb the XRay radiation and in turn emitvisible radiation capable of exciting the photosensitizer used fortreatment of various malignancies.

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The claimed invention is:
 1. A nanocrystal thin film comprising asuperlattice of assembled, monodisperse particles, the particles eachhaving: a single pure crystalline phase of a rare earth-containinglattice, a uniform three-dimensional size, and a uniform polyhedralmorphology.
 2. The nanocrystal thin film of claim 1, wherein the largestdimension of the particles ranges from about 1 nm to 1,000 nm.
 3. Thenanocrystal thin film of claim 1, wherein at least one dimension of theparticles ranges from about 1 um to 250 um.
 4. The nanocrystal thin filmof claim 1, wherein the rare earth-containing lattice further comprisesat least one lattice modifier.
 5. The nanocrystal thin film of claim 4,wherein the lattice modifier is an alkali metal and/or an alkaline earthmetal.
 6. The nanocrystal thin film of claim 1, wherein the particleshave crystal symmetries of tetragonal bipyramids, hexagonal prisms,rods, hexagonal plates, ellipsoids, trigonal prisms, or triangularplates.
 7. The nanocrystal thin film of claim 1, wherein the particlesare down-converting phosphor particles.
 8. The nanocrystal thin film ofclaim 7, wherein the down-converting phosphor particles are selectedfrom the group consisting of rare earth element doped oxides, rare earthelement doped oxysulfides, rare earth element doped fluorides, sodiumgadolinium fluorides doped with other lanthanides, and lanthanidefluorides.
 9. The nanocrystal thin film of claim 1, wherein theparticles are up-converting phosphor particles.
 10. The nanocrystal thinfilm of claim 9, wherein the up-converting phosphors are selected fromthe group consisting of sodium yttrium fluoride, lanthanum fluoride,lanthanum oxysulfide, rare earth oxysulfide, rare earth oxyfluoride,rare earth oxychloride, yttrium fluoride, yttrium gallate, gadoliniumfluoride, barium yttrium fluoride, and gadolinium oxysulfide.
 11. Thenanocrystal thin film of claim 1, wherein the particles are crystallinenanoparticles.
 12. The nanocrystal thin film of claim 1 wherein thesingle pure crystalline phase is a beta-phase.
 13. The nanocrystal thinfilm of claim 1, wherein the rare earth-containing lattice is anyttrium-containing lattice.
 14. The nanocrystal thin film of claim 13,wherein the yttrium-containing lattice is selected from YF₃, LiYF₄,NaYF₄, BaYF₃, BaY₂F₈NaYF₄, KYF₄, Y₂O₂S, and Y₂O₃.
 15. The nanocrystalthin film of claim 13, further comprising a dopant selected from La, Ce,Pr, Ne, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and mixturesthereof.
 16. The nanocrystal thin film of claim 1, wherein the rareearth-containing lattice is a lanthanide-containing lattice.
 17. Thenanocrystal thin film of claim 16, wherein the lanthanide-containinglattice is selected from LaF₃, CeF₃, PrF₃, NeF₃, PmF₃, SmF₃, EuF₃, GdF₃,TbF₃, DyF₃, HoF₃, ErF₃, TmF₃, YbF₃LuF₃, NaGdF₃, Gd₂OS₂, LiHoF₄, LiErF₄,CeO, SrS, CaS, and GdOCl.
 18. The nanocrystal thin film of claim 16,further comprising a dopant selected from Y, La, Ce, Pr, Ne, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and mixtures thereof, with the provisothat the dopant is not also the lattice lanthanide.
 19. An optical filmhaving incorporated therein the nanocrystal thin film of claim
 1. 20. Avolumetric display having incorporated therein the nanocrystal thin filmof claim 1.